Rubber composition for tire tread and pneumatic tire comprising the same

ABSTRACT

The present invention provides a rubber composition for a tire tread which can prepare a tire having improved grip properties under conditions of a high temperature, and a pneumatic tire comprising the same. The present invention relates to the rubber composition for a tire tread containing more than 5 parts by weight of a basic antioxidant based on 100 parts by weight of the diene rubber components, wherein a metallic compound is a metallic compound comprising (1) a metallic salt of an organic carboxylic acid or (2) an inorganic metallic salt and an acid, and the pneumatic tire having a tire tread comprising the same.

BACKGROUND OF THE INVENTION

The present invention relates to a rubber composition for a tire treadand a pneumatic tire comprising the same.

A part of a tread of a pneumatic tire had a problem of lowering gripproperties under the condition of high temperature by building up heatas driving.

Conventionally, to solve those problems, compounding a resin that has ahigh glass transition temperature into a rubber composition for a treadtire and also compounding an imidazole compound into a rubbercomposition for a tread tire have been conducted, however, gripproperties under the condition of a high temperature could not beimproved efficiently.

Japanese Unexamined Patent Publication No. 2003-213045 disclosed arubber composition for a tire containing an organic metallic compoundsuch as magnesium methacrylate or zinc methacrylate was disclosed,however, there was a problem that grip properties under the condition ofa high temperature could not be improved efficiently, since inhibitionof crosslinking occurred by isolating methacrylic acid in kneading theorganic metallic compound.

Patent reference: Japanese Unexamined Patent Publication No.2003-213045.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a rubber compositionfor a tire tread which can prepare a tire having improved gripproperties under conditions of a high temperature, and a pneumatic tirecomprising the same.

The present invention relates to a rubber composition for a tire treadcomprising a diene rubber component, a basic antioxidant and a metalliccompound, wherein the metallic compound is a metallic compoundcomprising (1) a metallic salt of an organic carboxylic acid or (2) aninorganic metallic salt and an acid, and the basic antioxidant iscontained in an amount of more than 5 parts by weight based on 100 partsby weight of the diene rubber component.

The metallic salt of an organic carboxylic acid (1) preferably does notcontain a multiple bond.

Also, the present invention relates to a pneumatic tire comprising arubber composition for a tread tire.

DETAILED DESCRIPTION

A rubber composition for a tire tread of the present invention comprisesa diene rubber component, a metallic compound and a basic antioxidant.

As a diene rubber, exmaples are a natural rubber (NR), astyrene-butadiene rubber (SBR), a butadiene rubber (BR), an isoprenerubber (IR), a butyl rubber, an acrylonitrile-butadiene rubber (NBR), anethylene-propylene rubber (EPDM) and a chloroprene rubber (CR). Amongthose, SBR, NR and BR are preferably used and SBR is more preferablyused in viewpoints of having sufficient strength and showing excellentabrasion resistance as a rubber for a tire tread.

The metallic compound is a compound comprising either (1) a metallicsalt of an organic carboxylic acid or (2) a metallic salt of aninorganic carboxylic acid and an acid (hereinafter referred to as themetallic acid(2)).

Those metallic compounds contain an ionic bond. In this manner, since itis possible that loss is produced (tan D is produced) with a highertemperature or greater strain by containing an ionic bond with strongerbonding forth than a hydrogen bond (a bond of a nitrogen compound and anacid), grip properties at a high temperature can be improved. Further,since the metallic compound have a small change of a glass transitiontemperature, a risk of brittle fracture becomes low.

As the metallic salt of the organic carboxylic acid (1), examples are anacetate salt, an acrylate salt, a methacrylate salt and a propionatesalt. Among those, since an acrylate salt and a methacrylate saltcontains a double bond, the acrylate aslt and a methacrylate saltcorrespond to the metallic salt of an organic carboxylic acid containinga multiple bond, and since a propionate salt does not contain a multiplebond such as a double bond, the propionate salt corresponds to themetallic salt of an organic carboxylic acid which does not contain amultiple bond.

Among the metallic salts of organic carboxylic acids which have amultiple bond and a methacrylate salt is preferable, and at least onekind selected from the group consisting of magnesium methacrylate andzinc methacrylate is more preferable since tan D at a high temperaturecan be obtained.

Also, among the metallic salts of the organic carboxylic acids, which donot contain a multiple bond such as a double bond, metallic salt ofacetate is preferable and magnesium acetate is more preferable sincetanD at a high temperature is obtained.

As the metallic salt of the organic carboxylic acid (1), a metallic saltof an organic carboxylic acid which does not contain a multiple bond ispreferable, rather than a metallic salt of an organic carboxylic acidwhich contains a multiple bond. Scattering of crosslinking is inhibitedand a density of crosslinking can be improved since a multiple bond isnot contained.

The amount of the metallic salt of the organic carboxylic acid (1) ispreferably at least 0.5 part by weight based on 100 parts by weight of adiene rubber component and more preferably 2 parts by weight. Also, theamount of the metallic salt of the organic carboxylic acid (1) ispreferably at most 20 parts by weight based on 100 parts by weight of adiene rubber component, and more preferably 10 parts by weight. If theamount of the metallic salt of the organic carboxylic acid (1) is morethan 20 parts by weight, tackiness tends to increase.

The metallic compound (2) comprises an inorganic metallic salt and anacid. As the inorganic metallic salt, examples are magnesium oxide andcalcium oxide. Also, as the acid, examples are an organic compoundcontaining a hydroxyl group, a carboxylic group and the like, andgeneral acids, more concretely, an acetic acid and a propionic acid.

With respect to the metallic compound (2), the inorganic metallic saltis compounded in such a large amount not to generate an acid, but thecontent ratio between acid and a inorganic metallic salt can becompounded to have an equivalent electric charge. For example, thecontent ratio between acetic acid and a magnesium oxide can be a molarratio of 2:1.

The amount of the inorganic metallic salt is preferably at least 0.2part by weight based on 100 parts by weight of a diene rubber componentand more preferably at least 0.7 part by weight. Also the amount of theinorganic metallic salt is preferably at most 7.5 parts by weight basedon 100 parts by weight of a diene rubber component and more preferablyat most 3.5 parts by weight. If the amount of the inorganic metallicsalt is more than 7.5 parts by weight, tackiness tends to increase.

The amount of the acid is preferably at least 0.5 part by weight basedon 100 parts by weight of the diene rubber component and more preferablyat least 2 parts by weight. Also the amount of the acid is preferably atmost 20 parts by weight based on 100 parts by weight of a diene rubbercomponent and more preferably at most 10 parts by weight.

A basic antioxidant is defined as an antioxidant showing basicproperties. The basic antioxidant is compounded to neutralize a highdegree of acidity caused by an organic acid isolating from the organicmetallic compound at kneading.

As a basic antioxidant, examples are secondary amines of aromaticseries, amine-ketones, benzimidazoles, and thioureas.

As the secondary amines of aromatic series, examples areN-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine,phenyl-α-naphthylamine, 4,4′-bis(α,α-dimethylbenzyl)diphenylamine,alkylated diphenylamine and (p-toluenesulfonylamido)diphenylamine.

As amine-ketones, examples are a 2,2,4-trimethyl-1,2-dihydroquinolinepolymer, 6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline, a condensate ofaniline and ketone and a condensate of diphenylamine and acetone.

As benzimidazoles, examples are 2-mercaptobenzimidazole,2-mercaptomethylbenzimidazole, a zinc salt of 2-mercaptobenzimidazole, azinc salt of 2-mercaptomethylbenzimidazole As thioureas, examples are1,3-bis(dimethylaminopropyl)thiourea and tributylthiourea.

Those basic antioxidants can be employed solely or in a combinationthereof, particularly a combination use of secondary amines of aromaticseries and amine-ketones is preferable and specifically a combinationuse of N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine and2,2,4-trimethyl-1,2-dihydroquinoline polymer is more preferable.

The amount of the basic antioxidant is more than 5 parts by weight basedon 100 parts by weight of a diene rubber component and preferably morethan 7 parts by weight. If the amount of the basic antioxidant is lessthan 5 parts by weight, a high degree of acidity cannot be neutralizedand inhibition of crosslinking occurs. Also the amount of the basicantioxidant is at most 20 parts by weight based on 100 parts by weightof a diene rubber component and preferably at most 10 parts by weight.

Other than the above mentioned rubber components, the metallic compoundand the basic antioxidant, the rubber composition for a tire tread ofthe present invention can suitably contain a reinforcing filler such ascarbon black and silica, a softener such as an aroma oil, a stearicacid, zinc oxide, and a vulcanizing agent such as sulfur and avulcanization accelerator, which are general additives employed in tireindustries.

It is general that a ordinary rubber composition for a tire tread isprepared by two kneading steps comprising the first step of kneadingchemicals except for a vulcanizing agent and a vulcanization acceleratorand the second step of adding the vulcanizing agent and thevulcanization accelerator into the obtained kneaded product and kneadingfurther. Also, the rubber composition for a tire tread of the presentinvention can be prepared by three kneading steps described below.

In the first step, the diene rubber, the reinforcing filler, the basicantioxidant, stearic acid, zinc oxide and the like are kneaded. In thesecond step, the softener, the metallic compound, the basic antioxidantand the like are kneaded. In the third step, the vulcanizing agent, thevulcanizing accelerator and the like are kneaded.

In this manner, an effect of preventing an acid isolating from themetallic compound from neutralizing can be obtained by which kneadingthe softener, the metallic compound and the basic antioxidant is in thesecond step, but there is no particular limitation in the presentinvention.

The pneumatic tire of the present invention is prepared by a generalprocess, employing the rubber composition for a tire tread as a tiretread. Namely, the above mentioned rubber composition isextrusion-processed into a form of a tread part of a tire in the stageof unvulcanizing and laminated on a tire molding machine by a generalprocess to mold the unvulcanized tire. The pneumatic tire is obtained byheating-pressuring the unvulcanized tire in a vulcanizer.

Hereinafter, the present invention is explained in detail based onExamples, but the present invention is not limited thereto.

Various chemicals used in Examples are described below.

SBR: Tufdene 4350 (the amount of a bonded styrene 39%, containing 50parts by weight of oil based on 100 parts by weight of a rubber solidcontent) available from ASAHI KASEI CORP.

Carbon black: DIABLACK A (N110) available from Mitsubishi ChemicalCorporation

Antioxidant 6C: Santoflex 13(N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine) available fromFLEXSIS CO.

Antioxidant 224: NOCRAC 224 (2,2,4-trimethyl-1,2-dihydroquinolinepolymer) available from FLEXSIS CO.

Stearic acid: Stearic acid available from NOF Corporation

Zinc oxide: Zinc Oxide type 2 available from Mitsui Mining and SmeltingCo., Ltd.

Aroma oil: Process X-260 available from JAPAN ENERGY CORP.

Magnesium methacrylate: SK-13 available from SANSHIN CHEMICAL INDUSTRYCO., LTD

Magnesium acetate: available from KISHIDA CHEMICAL CO., LTD

Acetic acid

Magnesium oxide

Sulfur: Powdery sulfur available from Tsurumi Chemicals Co., Ltd.

Vulcanization accelerator: Nocceler NS available from Ouchi ShinkoChemical Industrial Co., Ltd.

EXAMPLES 1 to 5 and COMPARATIVE EXAMPLES 1 to 4

(Process for Preparation of a Rubber Sample of Example 1 to 2 andComparative Example 1 to 3)

According to the amounts shown in Table 1, other than an aroma oil,magnesium methacrylate, sulfur and the vulcanization accelerator,various chemicals were base-kneaded in a BP banbury mixer, thereto wereadded an aroma oil, magnesium methacrylate and the antioxidant 6C andthe mixture was base-kneaded for 4 minutes and discharged at atemperature of 150° C. to obtain the kneaded product. To the kneadedproduct were added sulfur, the vulcanization accelerator and the mixturewas kneaded for 5 minutes by using an open roll. A sheet was preparedwith the obtained rubber composition and rubber samples of Example 1 to2 and Comparative Example 1 to 3 were prepared by vulcanizing with aspecified mold for 12 minutes at a temperature of 170° C. Also,re-rolling was conducted in Example 2 and Comparative Example 2. Herein,re-rolling is defined that the kneaded rubber in a banbury mixer wasrolled and gave stimulation by conducting heating to isolate an acid.

(Process for Preparation of Rubber Samples of Examples 3 to 5 andComparative Example 4)

According to the amounts shown in Table 2, other than an aroma oil,magnesium methacrylate, magnesium acetate, acetic acid, magnesium oxide,sulfur and the vulcanization accelerator, various chemicals were kneadedfor 3 minutes in a BP banbury mixer, thereto were added an aroma oil,magnesium methacrylate, magnesium acetate, acetic acid, magnesium oxideand an antioxidant 6C and the mixture were base-kneaded for 3 minutesand discharged at a temperature of 150° C. to obtain the kneadedproduct. To the above mentioned kneaded product were added sulfur andthe vulcanization accelerator and the mixture was kneaded for 5 minutesby using an open roll. A sheet was prepared with the obtained rubbercomposition and a rubber sample of Examples 3 to 5 and ComparativeExample 4 were prepared by vulcanizing with a specified mold for 12minutes at a temperature of 170° C.

The following tests were conducted by using the obtained rubber samples.

(Degree of Crosslinking (SWELL))

SWELL was evaluated by toluene-extracting the rubber samples. The largerthe measurement value is, the greater the scattering of crosslinking is,which does not mean preferable.

(Viscoelasticity)

10% of initial strain was given by using the viscoelasticityspectrometer made by Iwamoto Corporation and viscoelasticity (complexmodulus E′ and loss coefficient tanD) under the condition of giving 2%of dynamic strain at 100° C. was measured. The larger tanD index is, thehigher the grip is, which means that grip properties is excellent.

(Tensile Test)

In accordance with JIS Tensile Test 6251, Dumbbell No. 3 sample wastested and an the index was respectively represented, regarding theindex of Example 1 or Example 4 as 100. The greater M300 (stress at 300%elongation) is, the more abrasion properties becomes.

(Process for Preparation of Tires of Example 1 and Comparative Examples1 to 2)

According to the amounts shown in Table 1, other than an aroma oil,magnesium methacrylate, sulfur and the vulcanization accelerator,various chemicals were base-kneaded for 3 minutes in a BP banbury mixer,thereto were added an aroma oil, magnesium methacrylate and theantioxidant 6C and the mixture was base-kneaded for 4 minutes anddischarged at 150° C. to obtain the kneaded product. To the abovementioned kneaded product were added sulfur and the vulcanizationaccelerator, and the mixture was kneaded for 5 minutes by using an openroll to prepare a sheet with the obtained rubber composition and thesheet were laminated in the specified form to prepare 11×7. 10-5 size ofcart tires of Example 1 and Comparative Examples 1 to 2.

(Process for Preparation of Tires of Examples 3 to 5 and ComparativeExamples 4)

According to the amounts shown in Table 2, other than an aroma oil,magnesium methacrylate, magnesium acetate, acetic acid, magnesium oxide,sulfur and the vulcanization accelerator, various chemicals werebase-kneaded for 3 minutes in a BP banbury mixer, thereto were added anaroma oil, magnesium methacrylate, magnesium acetate, acetic acid,magnesium oxide and the antioxidant 6C, and the mixture was base-kneadedfor 4 minutes and discharged at 150° C. to obtain the kneaded product.To the above mentioned kneaded product were added, sulfur and thevulcanization accelerator, and the mixture was kneaded for 5 minutes byusing an open roll to prepare a sheet with the obtained rubbercomposition and the sheet was laminated in the specified form to prepare11×7. 10-5 size of cart tires of Examples 3 to 5.

The obtained cart tire was mounted on an automobile and the runningevaluation was conducted.

(Grip Test)

The above mentioned tire was mounted on a cart and evaluated by driving8 rounds of 2 km round circuit course. Tire grip feeling was evaluated,assuming that of Comparative Example 1 as 3 points according to themaximum point 5. Initial grip was measured at 1 to 4 round and thelatter grip at 5 to 8 round.

(Abrasion Appearance)

The above mentioned tire was mounted on a cart and evaluated by driving8 rounds of 2 km round circuit course. Relative evaluation wasconducted, assuming appearance of the tire of Comparative Example 1 as 3points according to 5 maximum points.

Each evaluation result is represented in Table 1 and 2. TABLE 1 Ex. Com.Ex. 1 2 1 2 3 Amount (parts by weight) SBR 150 150 150 150 150 Carbonblack 100 100 100 100 100 Antioxidant 6C 2.5 2.5 2.5 2.5 2.5 Antioxidant224 1.5 1.5 1.5 1.5 1.5 Stearic acid 2 2 2 2 2 Zinc oxide 4 4 4 4 4Aroma oil 45 45 50 50 50 Magnesium methacrylate 5 5 0 5 5 Antioxidant 6C5 5 0 0 0 Sulfur 1.2 1.2 1 1.2 1.2 Vulcanization 2.5 2.5 2.5 2.5 2.5accelerator Total amount of 9 9 4 4 4 antioxidants (parts by weight)Rerolling — ◯ — — ◯ Evaluation results SWELL 225 224 223 230 256Viscoelasticity E′ 3.0 2.8 3.0 3.1 2.8 tanD 0.33 0.34 0.31 0.35 0.34Tensile test M300 98 99 100 98 95 Running evaluation Initial gripevaluation 3 — 3 3 — Latter grip evaluation 3.5 — 3 3 — Abrasionappearance 3 — 3 2.5 —

TABLE 2 Ex. Com. Ex. 3 4 5 4 Amount (parts by weight) SBR 150 150 150150 Carbon black 100 100 100 100 Antioxidant 6C 2.5 2.5 2.5 2.5Antioxidant 224 1.5 1.5 1.5 1.5 Stearic acid 2 2 2 2 Zinc oxide 4 4 4 4Aroma oil 45 45 45 50 Magnesium methacrylate 5 — — — Magnesium acetate —5 — — Antioxidant 6C 5 5 5 — Acetic acid — — 5 — Magnesium oxide — — 1.5— Sulfur 1.2 1 1 1 Vulcanization accelerator 2.5 2.5 2.5 2.5 Totalamount of antioxidants 9 9 9 4 (parts by weight) Evaluation resultsSWELL 230 221 223 223 Viscoelasticity E′ 2.6 3.1 3.3 3.0 tanD 0.35 0.340.35 0.31 Tensile test M300 80 98 102 100 Running evaluation Initialgrip evaluation 3 2.5 2.5 3 Latter grip evaluation 4 4 4 3 Abrasionappearance 2 3.5 3.5 3

According to the present invention, inhibition of crosslinking caused byacid generated at kneading is restrained by kneading a certain amount ofa particular metallic compound and a basic antioxidant, and furthermore,a pneumatic tire prepared as a tire tread from a rubber compositionobtained by kneading can show excellent grip properties in anenvironment of a high temperature.

1. A rubber composition for a tire tread comprising diene rubbercomponents, a basic antioxidant and a metallic compound, wherein themetallic compound is a metallic compound comprising (1) a metallic saltof an organic carboxylic acid or (2) an inorganic metallic salt and anacid, and the basic antioxidant is contained in an amount of more than 5parts by weight based on 100 parts by weight of the diene rubbercomponents.
 2. The rubber composition of the tire tread of claim 1,wherein the metallic salt of the organic carboxylic acid (1) does notcontain a multiple bond.
 3. A pneumatic tire having a tire treadcomprising the rubber composition for the tire tread of claim 1 or 2.